Organic EL devices which utilize organic substances are expected to be useful for application as an inexpensive full color display device of the solid light emission type having a great size, and various developments on the organic EL devices are being conducted. In general, an organic EL device has a construction comprising a pair of facing electrodes and a light emitting layer sandwiched between the electrodes.
The light emission of the organic EL device is a phenomenon in which, when an electric field is applied across the two electrodes, electrons are injected from the cathode side and holes are injected from the anode side, the electrons recombine with the holes in the light emitting layer to induce an excited state, and then, when the excited state returns to the original state, it emits energy as light.
As the light emitting material, chelate complexes such as tris(8-quinolinolato)aluminum, coumarine derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives and oxadiazole derivatives are known. It has been reported that these light emitting materials emit light in the visible region of blue to red, and it is expected that color display elements can be obtained by using these light emitting materials (see, for example, Japanese Patent Application Laid-Open Nos. Heisei 8 (1996)-239655, Heisei 7 (1995)-138561 and Heisei 3 (1991)-200889).
Although the practical use of displays using organic EL devices recently started, the full color display device is still under development. In particular, an organic EL device which emits bluish light with excellent purity of color and great efficiency of light emission has been desired.
To overcome the above problems, for example, a device in which a phenylanthracene derivative is used as the material emitting blue light is disclosed in Japanese Patent Application Laid-Open No. Heisei 8 (1996)-12600. The phenylanthracene derivative is used as the material emitting blue light and, in general, used as a laminate of a layer of the material emitting blue light with a layer of a complex of tris(8-quinolinolato)aluminum (Alq). However, the efficiency of light emission, the life and the purity of blue light are insufficient for the practical application. Japanese Patent Application Laid-Open No. 2001-160489 discloses a device in which an azafluoranthene compound is added to the light emitting layer. However, this device emits yellow to green light and cannot emit blue light having a sufficiently high purity of color.